The present invention pertains to a photomask drying device used especially in the manufacture of submicron-sized microelectronic components. It can furthermore be extended to the method for drying the photomask.
The micromachining of semi-conductor substrates, especially made of silicon, is currently being done by a plasma-etching techniques used to etch a pattern transferred to the substrate from a photomask by a lithography method. A photomask is equivalent to a negative in photography; it contains a piece of information to be printed on a medium. The pollution in the active zone of the photomask has a direct effect on the image printed on the substrate in that a defect is printed.
Moreover, the semiconductor industry is seeking to reduce the size of the recorded image in order to obtain ever smaller, increasingly integratable and ever less costly electronic components. With the dimensions of a photomask getting smaller, the requirements in terms of pollution are becoming increasingly stringent, in order to enable the etching of patterns with dimensions smaller than 100 nm, the wavelength of the lithography laser has gone from 193 nm in the prior-art technique to 13.5 nm in the more recent technique known as the EUV (extreme ultra-violet) technique. The photomask is therefore a vital, costly and complex element that has to be kept clean and operational.
At the end of its manufacture, the photomask is cleaned. For a photomask used in classic techniques, a film is then applied to the photomask in order to protect its active face from any particles. The final cleaning step is done in a wet medium. Following the cleaning step, the photomask has to be carefully dried in order to eliminate all residues of adsorbed humidity. Today, if is by applying heat at very high temperatures (of the order 80-90° C.) that humidify is usually eliminated. This is a mode of drying that acts essentially on the surface.
An EUV photomask designed to be used in the EUV technique carries no film. If is a fiat element formed by a “multilayered” type of stacking formed by very fine layers of different materials (MoSi, TaN etc) deposited on a quartz base. It cannot be exposed to a temperature of over 60° C. or to a temperature variation of more than 10° C. on its active surface because an excessively high temperature or excessively steep temperature gradient would lead to an excessively great expansion of certain layers, and the EUV photomask would thereby deteriorate.
The efficiency of the drying is generally assessed by measuring the angle of contact θ of a drop of liquid on the surface of the photomask. A drop is deposited on a solid surface. It spreads out until an equilibrium of forces is attained between the different phases present (solid, liquid and vapor). The relationship between these different forces and the angle of contact is given by Young's equation:γSV=γLV cos θ+γSL wherein:                γSV is the interfacial tension between solid and vapor,        γLV is the interfacial tension between liquid and vapor,        γSL is the interfacial tension between solid and liquid, and        θ is the value of the angle of contact between a drop of liquid and the surface of the photomask.        
The hydrophilic or hydrophobic character of the surface of a photomask depends on the material that constitutes it, and is measured by the angle of contact θ. Once the surface of the photomask is truly dry, it becomes hydrophobic, the spread of the drop is limited and the angle of contact is big. In order that the surface of a photomask may be considered to be sufficiently hydrophobic, the angle of contact θ must be greater than 55° for chromium, 25° for quartz, 80° for ruthenium and 70° for tantalum nitride. Now, the methods that use heating, such as the ones currently used, cannot achieve this degree of efficiency while remaining at moderate temperatures.
The need is being felt for a drying method that meets both the constraints related to the EUV photomask itself and the constraints related to already existing equipment and manufacturing conditions. However, the drying obtained by this method must be equally efficient or even more efficient than that obtained by known methods. The drying step must enable a homogenous drying in volume of the EUV photomask without causing its deterioration, especially because of the temperature, and without disturbing the manufacturing process. Thus, the EUV photomask has to be dried within a maximum time of 20 minutes. This corresponds to the time available in the manufacturing flow. At the end of the drying step, the EUV photomask should be at a temperature of the order of 30° C. to 35° C.
It is also an aim of the present invention to propose a method for drying an EUV photomask that is to be used in the EUV technique, is film-free, and for which the drying temperature must not exceed 60° C.
It is yet another aim of the invention to propose a device for ensuring the homogeneity of the temperature throughout the surface of the EUV photomask. Indeed, the temperature has to be controlled with high precision because the temperature on the surface of the EUV photomask should be kept constant and uniform with a maximum variation of ±5° C. throughout the duration of the drying stop.